In surgery, there are occasions when cavities formed in a bone are filled by a mass consisting of granules of bone cement impregnated with the patient's own blood. For this filling step, a granules syringe is commercially available from Orthomatrix, Inc., which includes a transparent barrel for receiving the granules between a front cap and a piston reciprocally disposed within the barrel. The cap is provided with a screen to allow the impregnating liquid to pass while retaining the granules. A piston rod extending beyond the open rear end of the barrel permits the piston to be manually retracted and advanced.
In use, the syringe is immersed with its front cap into blood taken from the patient, and the blood is drawn in by retracting the piston. By shaking the device, the granules are subsequently mixed with the blood. For an efficient mixing, the volume of the chamber must be greater than the total volume occupied by the granules and the blood. In other words, by retracting the piston air must be taken in, in addition to blood. When the mixing step has been completed, this air must be expelled by advancing the piston, before the mixture of bone cement and blood may be applied to the bone cavity by further advancement of the piston.
A problem with the known granules syringe resides in the fact that a thorough impregnation of the granules is difficult to achieve. The efficiency of the manual shaking movement depends not only on the skill and patience of the operator but also by the additional amount of air taken into the mixing chamber. Since the front cap is permeable to blood, there is a limit to the vehemence of the shaking movement, unless a further closure is provided.
Another serious disadvantage of the known apparatus resides in the fact that air is introduced into the mixture due to the shaking movement, which air impairs the quality of the finished bone filling mass and which can be removed only in part and only with considerable effort.
DE-3,701,190-A discloses a device for sucking in or discharging liquid or pasty masses including bone cement, in which a piston is sealed with respect to a cylinder by means of a lip seal surrounding the piston so that air trapped in the mass within the cylinder can be withdrawn by a vacuum source connected to the cylinder behind the piston.